1. Field of the Invention
The present invention relates to a high-efficiency regulated voltage-boosting device which may be used in particular for driving liquid-crystal displays (LCDs).
2. Description of the Related Art
As is known, regulated voltage boosters are used when it is necessary to supply one part of a circuit or device with a boosted voltage higher than the normal supply voltage. These regulated voltage boosters are based upon charge-pump circuits which comprise a preset number of voltage-boosting stages with negative feedback. In particular, the number of voltage-boosting stages required, which defines the multiplication factor of the charge pump, depends upon the ratio between the supply voltage and the boosted voltage that it is necessary to obtain. The number of voltage-boosting stages must moreover be chosen in such a way that the nominal voltage levels are guaranteed even in the worst operating conditions. In many cases, in fact, the ratio between the supply voltage and the boosted voltage is not constant, but varies in time. For example, it is known that in battery-supplied devices, the supply voltage may fluctuate and tends to decrease even to a considerable extent as the battery charge runs out. Consequently, in case of operation with low battery charge, a higher multiplication factor is required than in case of fully charged battery.
In known regulated voltage boosters, on the other hand, when the charge-pump circuit is activated for regulating the boosted voltage, all the voltage-boosting stages are in any case operated simultaneously, irrespective of the operating conditions.
The above is clearly disadvantageous, since the efficiency is considerably penalized, above all when the boosted regulator operates in nominal conditions (in which a low multiplication factor is sufficient).
For greater clarity, reference is made to the example of a boosted regulator for driving LCDs, in which a lithium battery supplies a supply voltage VDD ranging between 2.8 V and 4.2 V. The required driving voltage VLCD depends upon the number of rows present in the display and is normally higher than the supply voltage, for instance VLCD (12 V). In the worst conditions (VDD=2.8 V), the multiplication factor M of the charge pump must be at least 5. In fact, in this case we have VLCD=VDD*M=2.8*5=14 V. Since M (1 voltage-boosting stages are required, in general, in order to reach a multiplication factor equal to M, the charge pump of the example must comprise four stages. However, most of the time the regulated voltage booster is operated in conditions close to the nominal conditions, in which VDD=4.2 V. In this case, a multiplication factor M equal to 3 is enough; in fact we have VLCD=VDD*M=4.2*3=12.6 V. In practice, just two voltage-boosting stages would normally be sufficient, but in order to meet a greater variety of operative conditions, four stages must be provided which are all actuated whenever the charge pump intervenes. As already mentioned, this entails considerable power consumption, and hence a low level of efficiency.
An embodiment of the present invention provides a regulated voltage booster that has dynamic allocation of boost voltage and, in particular, ensures high levels of efficiency.
According to an embodiment of the present invention, a regulated voltage booster and a method for controlling a regulated voltage-boosting device are provided. The regulated voltage-boosting device includes a charge-pump circuit, which has an input terminal for receiving a first voltage (VDD) and an output terminal for supplying a second voltage (VLCD) higher than the first voltage (VDD). The device provides a plurality of voltage-boosting stages that can be selectively activated and deactivated. The device includes an automatic-selection circuit for activating a number of the voltage-boosting stages, which is correlated to the existing operating conditions. Also, the method controls a regulated voltage-boosting device having a charge-pump circuit to provide a plurality of voltage-boosting stages which can be selectively activated and deactivated. The method including steps of supplying a first voltage (VDD) to the charge-pump circuit; generating a second voltage (VLCD) higher than the first voltage (VDD) such that the activation of a number (M) of the voltage-boosting stages is correlated to existing operating conditions of the device.